1. Field of the Invention
The present invention relates to biocompatible medical devices such as sutures and the like which also may be absorbable and to methods of making them.
2. Description of the Background Art
The advantages of absorbable materials in surgical applications are universally appreciated. The traditional naturally derived suture, known as xe2x80x9ccatgut,xe2x80x9d is formed from collagenous material obtained from sheep or beef intestine. More recently, synthetic absorbable sutures of varying chemical composition have been developed.
A number of synthetic polymers have been described for use in making sutures and other bioresorbable medical devices. Effective synthetic absorbable sutures, as well as other medical devices such as haemostatic aids, intraosseous implants, slow-release drug delivery systems, and tissue regeneration devices including nerve channels, sperm ducts, vascular graphs, Fallopian tube ducts and the like, must satisfy a number of biological, physical and chemical requirements. Among these requirements are that the material be bioresorbable, non-carcinogenic, non-antigenic, and non-toxic.
Further, satisfactory bioresorbable polymers for medical applications need to have appropriate mechanical properties including flexibility, tensile strength, dimensional stability, should be sterilizable and absorbable by living tissue at a uniform rate. With respect to sutures, flexibility, adequate straight tensile and knot strength and the capability of being properly and easily tied in surgical knots are particularly desirable characteristics.
Various synthetic polymers have been proposed for use in the fabrication of sutures and other medical devices. Of particular interest are homopolymers and especially copolymers of lactic acid and glycolic acid. Such copolymers have been developed in an attempt to combine the characteristics of both compounds and extend the range of polymer properties and rates of hydrolysis. For example, poly-L-lactic acid is hydrolyzed more slowly than polyglycolic acid and copolymers of the two acids can be made to hydrolyze at intermediate rates. Polymers of this type, and their use in the preparation of synthetic absorbable sutures, are disclosed, for example, in U.S. Pat. Nos. 2,703,316, 3,468,853, 3,565,869, 3,636,956, 4,137,921, 4,744,365, 4,839,130 and 5,124,103. Improved braided sutures, which may be composed of lactic acid and glycolic acid copolymers, are described in U.S. Pat. Nos. 5,019,093 and 5,037,429.
The use of lactic acid and glycolic acid copolymers in the manufacture of molded medical devices such as, for example, staples or clips is described in U.S. Pat. No. 4,523,591, which describes important and desirable properties for such molded articles. That patent also discloses procedures for injection molding, and other suitable molding techniques are known and employed in the art.
U.S. Pat. No. 3,736,646 discloses sterile synthetic copolymers containing lactic acid and glycolic acid having enhanced tissue absorption and solubility in organic solvents. That patent also contains reference to a number of other U.S. patents and publications which describe various approaches to the manufacture and use of synthetic polymeric sutures formed from lactic acid and glycolic acid.
Methods of preparing polymers of lactic acid and glycolic acid are described in the patents referred to above. These traditional chemical synthetic methods typically involve the use of a polymerization catalyst which, when combined with appropriately prepared monomer under specified atmospheric and temperature conditions, catalyses the formation of the polymer.
Of course, the way in which a polymer, and especially a copolymer, is made will affect the working characteristics of the resulting suture or other medical device. For example, U.S. Pat. No. 5,066,772, which discloses copolymers of recurring units derived from carbonates, lactides and glycolides, discloses copolymers which can be random copolymers or block copolymers, depending upon the properties desired. Random copolymers are disclosed as preferred where soft, pliable and relatively fast bioresorbable materials are required. Block copolymers are disclosed as preferred where hard, crystalline and relative slow bioresorbing materials are required. The patent contains an extensive description of block copolymers and the manner in which the selection of repeating block units may affect properties of the copolymer such as elasticity, modulus, pliability, hardness, softness, crystallinity and bioresorption rate.
U.S. Pat. No. 4,137,921 discloses a two-stage polymerization process for the preparation of lactic acid and glycolic acid copolymers. The first stage involves a random copolymerization of optically active lactic acid and glycolic acid monomer by conventional means. A second stage consists of further polymerization of the first stage polymer with additional lactic acid and glycolic acid monomer.
One drawback of traditional synthetic methods of producing polymers, such as those set forth in the U.S. patents referred to above, is that they often involve extreme reaction conditions. These include temperatures as high as 180xc2x0 C. for extended periods of time, use of highly volatile organic solvents such as chloroform and toluene, dry nitrogen reaction atmospheres and high vacuum. Further, these methods require the use of catalysts, some of which may be scarce commodities.
Perhaps the most important disadvantage of prior methods for making synthetic polymers is that they do not allow a high degree of control over the ultimate makeup of the polymer. Traditional chemical synthetic methods of making random copolymers, for example, rely upon crude adjustment of starting material ratios that can, at best, produce a polymer falling somewhere within a broad range of desired characteristics. Similarly, known methods of producing block copolymers are relatively crude, and have the additional disadvantage of requiring tedious and expensive chemical reaction steps.
Copolymer formation also is complicated by the fact that the relative rates of reactivity of glycolide and lactide are different. For example, when equimolar amounts of glycolide and lactide are reacted, glycolide is initially more likely to combine with growing chains than is lactide. Consequently, the initial composition of the growing chain contains a predominance of glycolic acid units occasionally and randomly interspersed with short sequences of lactic acid units. As the reaction proceeds, the concentration of lactide contained in the mixture increases relative to glycolide, and the ratio of glycolic acid units to lactic acid units forming the chain becomes more equal. As the reaction nears completion, most available glycolide has polymerized and the relative amount of lactide is high. Consequently, a larger number of lactic acid units are likely to come together and polymerize.
One consequence of this stoichiometric effect is that the first portion of the copolymer chain is likely to contain a predominance of glycolic acid units, and the end portion of the chain is likely to contain a predominance of lactic acid units. Random sequences generated by the synthesis of poly(lactide-co-glycolide) result in the formation of heterogeneous polymers, i.e., no two polymeric chains are likely to be identically duplicated. Consequently, the physical and chemical properties of such copolymers have been difficult to predict or control with a high degree of precision.
Obviously, optimal control of the properties of a synthetic copolymer material would be attained where the identity of each successive co-monomeric unit was individually and specifically determined from the very outset of the process. It can readily be seen that this would allow an exquisite degree of control, leading to singularly improved biocompatible and absorbable sutures and other medical devices. However, no such method has been described.
Accordingly, it is an object of the present invention to provide improved methods of making lactic acid and glycolic acid copolymers which allow each successive lactic acid or glycolic member of a polymeric chain to be individually and specifically specified. It is another object of the present invention to provide biocompatible and absorbable sutures and other medical devices comprised of lactic acid and glycolic acid copolymers made according to the methods of the invention.
It is a further object of the present invention to allow for the incorporation of individual or multiple amino acids into the polymers made according to the methods of the invention. These can include short or long lengths of amino acids which may have desirable bioactive characteristics. Short or long amino acid sections incorporated into the polymers of the invention may, for example, allow cell attachment, act as growth factors, or prevent thrombosis. Thus, another object of the invention is to provide synthetic copolymer compositions having incorporated therein one or more bioactive elements.
These and other objects of the present invention will be apparent to those of skill who appreciate and understand the teachings of the present specification, set forth in the following description.
The present invention is directed to biocompatible medical devices, including sutures, produced from polyesters formed by novel synthetic methods for the template driven synthesis of lactic acid and glycolic acid copolymers of defined sequence. The biocompatible medical devices of the invention also may be absorbable. The novel synthetic methods of the invention allow each successive lactic acid or glycolic acid member of the copolymer to be individually specified. This capability provides an unprecedented degree of control over the design and properties of the copolymer product. As a result of the present invention, greatly improved sutures and other medical devices can now be developed and produced.